The present invention relates to an optical lens and more particularly to a plastic lens which is adapted to be used in an optical system.
Recently, plastic lenses have been widely used in optical systems, e.g. photographic camera or laser system. At first, the conventional technic to mount a plastic lens in a lens mount will be explained referring to FIG. 1-a to FIG. 1-e.
Referring to FIG. 1-a, to mount a plastic lens 7 in a lens mount 2, the lens 7 is fit in a fitting portion 5 of the lens mount 2 and is urged on a shoulder 4 which is formed by a stepped portion in the lens mount 2. Then a hold ring 1 which has outer thread 6 is threaded in inner thread 3 of the lens mount 2 and is urged against the lens 7 by an abutment 8 which is formed by an inner edge of the ring 1. The clearance between the outer periphery of the lens 7 and the inner surface of the fitting portion 5 is very small to ascertain an accurate optical axis. As the abutment 8 of the ring 1 is urged directly on the lens surface, there is no clearance.
When the assembled lens mount 2 is heated by the atmosphere to a relatively high temperature, the lens 7 and the lens mount 2 expand by thermal expansion. As the linear expansion cofficient of the plastic lens 7 is larger than that of the lens mount 2, the lens 7 expands relative to the lens mount 2 and the clearance at the fitting portion 5 decreases. Also, as the initial clearance between the abutment 8 and the lens surface is zero, the thermal expansion of the lens 7 results in deformation of the lens. As shown in FIG. 1-b, the abutment 8 of the ring 1 digs into the lens surface and the internal diameter of the ring 1 holds the lens 7 at zero clearance.
Thus, when the ambient temperature is increased, the lens expands radially relative to the lens mount. However, the radial expansion is limited by the fitting portion 5 and/or abutment 8 of the ring 1, so that heat stress is produced in the lens. Consequently, the lens deforms its optical axis. As shown in FIG. 1-c, as the lens cannot expand normally to the optical axis, the lens deforms from line APB to line AP'B. Consequently, the radius of curvature of the lens decreases from r.sub.1 to r.sub.2.
When the ambient temperature decreases from the assembled temperature, the lens contracts relative to the lens mount. However, as the lens is held tightly by the abutment 8 of the ring 1, the lens cannot contract radially. Thus, as shown in FIG. 1-d the lens deforms from line APB to line AP'B, and the radius of curvature of the lens increases from r.sub.1 to r.sub.3.
As described, when the plastic lens is mounted in a conventional lens mount, the radius of curvature of the lens varies according to ambient temperature so that deviation of the focus position and other adverse effects occur.
FIG. 1-e shows two lenses 21 and 22 inserted by an alignment member 24 which acts to decrease the friction between the two lenses. As a hold ring 25 urges tightly the lenses 21 in a lens mount 23, above mentioned adverse effect at least at higher temperature occurs.